The present invention is directed to products made from unbleached pulps, the process used to make such products and the processes used to make cellulose solutions from unbleached pulp for spinning into lyocell products.
Cellulose is a polymer of D-glucose and is a structural component of plant cell walls. Cellulose is especially abundant in tree trunks from which it is extracted, converted into pulp, and thereafter utilized to manufacture a variety of products. Rayon is the name given to a fibrous form of regenerated cellulose that is extensively used in the textile industry to manufacture articles of clothing. For over a century, strong fibers of rayon have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process, cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. The cellulose is then reacted with carbon disulfide to form cellulose xanthate, which is then dissolved in a dilute caustic soda solution. After filtration and deaeration, the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting viscose rayon is presently used in textiles and was formerly widely used for reinforcing rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammonia copper oxide. This property forms the basis for the production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers, which are then decoppered and washed. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
The foregoing processes for preparing rayon both require that the cellulose be chemically derivatized or complexed in order to render it soluble and therefore capable of being spun into fibers. In the viscose process, the cellulose is derivatized, while in the cuprammonium rayon process, the cellulose is complexed. In either process, the derivatized or complexed cellulose must be regenerated and the reagents used to solubilize it must be removed. The derivatization and regeneration steps in the production of rayon significantly add to the cost of this form of cellulose fiber. Consequently, in recent years attempts have been made to identify solvents that are capable of dissolving underivatized cellulose to form a dope of cellulose from which fibers can be spun.
One class of organic solvents useful for dissolving cellulose are the amine N-oxides, in particular the tertiary amine N-oxides. For example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a cellulose fiber precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. Several manufacturers presently produce lyocell fibers, principally for use in the textile industry. For example, Acordis, Ltd. presently manufactures and sells a lyocell fiber called Tencel(copyright) fiber.
Currently available lyocell fibers are produced from wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose, and lignin. These highly processed pulps are referred to as high alpha (or high xcex1) pulps, where the term alpha (or xcex1) refers to the percentage of cellulose. Thus, a high alpha pulp contains a high percentage of cellulose, and a correspondingly low percentage of other components, especially hemicellulose and lignin. The processing required to generate a high alpha, low lignin pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom.
Furthermore, the wood chips are pretreated with an acid before the pulping stage, since it is not possible to obtain acceptable high alpha pulps for lyocell products otherwise through the Kraft process. A significant amount of material, primarily hemicellulose on the order of 10% or greater, of the original wood substance is solubilized in this acid phase pretreatment. Thus process yields are significantly diminished. Omitting the acid phase pretreatment will result in a high hemicellulose pulp. The disadvantage of conventional high alpha pulps is the reduction of yield by eliminating hemicelluloses from the pulp.
Conventional high alpha pulps are also bleached. Bleaching refers to the removal of lignin in a process subsequent to the pulping process. Removing lignin also reduces the overall yield of the original wood material.
In view of the expense of producing lyocell products from high alpha pulps that have small amounts of hemicellulose and lignin, it would be desirable to have alternatives to high alpha, low lignin pulps for making lyocell products.
Thus there is a need for relatively inexpensive, low alpha, high yield, high hemicellulose, and high lignin pulps that are useful for making lyocell products.
In U.S. Pat. No. 6,210,801, fully incorporated herein by reference in its entirety, high hemicellulose containing pulp is described that is useful for lyocell products. The pulp is made by reducing the viscosity of the cellulose without substantially reducing the hemicellulose content, followed by reducing the copper number.
While the methods described in the ""801 patent are effective at reducing the viscosity of cellulose without substantially decreasing the hemicellulose content, a further need existed for a process that did not require a separate copper number reducing step, and that was readily adaptable to pulp mills that have oxygen reactors. In U.S. Pat. No. 6,331,354, fully incorporated herein by reference in its entirety, a high hemicellulose, low viscosity pulp is described that is useful for making lyocell products that does not require an additional copper number reducing step. The pulp is made from an alkaline pulp by treating the alkaline pulp with an oxidizing agent in a medium to high consistency oxygen reactor to reduce the viscosity of the cellulose, without substantially reducing the hemicellulose or increasing the copper number of the pulp.
Further efforts to reduce the cost of making lyocell products are described in U.S. application Ser. No. 09/842,274, now U.S. Pat. No. 6,605,350 fully incorporated herein by reference in its entirety. In the ""274 application, the pulps are made from sawdust and other low length fiber wood. These pulps are high in hemicellulose and low in viscosity, and are composed of short fibers suitable for producing lyocell products.
However, until now, all of the prior art dissolving pulps for producing lyocell products are low in lignin content. It would be advantageous to develop a high lignin pulp that is useful for making lyocell products as an alternative to the highly refined low yield high alpha pulps.
In accordance with the present invention, lyocell products can be made with unbleached pulps resulting in products with high amounts of hemicellulose and high amounts of lignin as compared to conventional lyocell products. The lyocell products of the present invention are advantageously less expensive to produce but retain the desirable strength of conventional lyocell products.
One embodiment of the invention provides a high hemicellulose, high lignin lyocell fiber. The fiber has a dry tenacity of at least 46 cN/tex at a gauge length of about 10 mm and a dry tenacity of at least 30 cN/tex at a gauge length of about 30 mm. The hemicellulose content of the fiber is at least 7% and the lignin content of the fiber is at least 2% measured as Klason lignin.
In another embodiment of the invention, a lyocell fiber has an average diameter of from 9 to 16 microns; however, other embodiments of fibers have an average diameter of from 12 to 14 microns.
In another embodiment of the invention, a method for making a lyocell product is described. The method includes modifying a pulp having at least 7% hemicellulose and at least 2% Klason lignin by acid hydrolysis to lower the average degree of polymerization of the cellulose in the pulp, wherein greater than 95% of cellulose has an average D.P. from greater than about 400 to about 1100. The method also includes dissolving the unbleached pulp in a solvent to provide a cellulose solution, and spinning the cellulose solution into a lyocell product. The method uses meltblowing, centrifugal spinning, spun-bonding, or dry-jet wet spinning techniques. Fibers, films, and self-bonded nonwoven webs from unbleached pulps can be produced according to the methods of the invention.
In another embodiment of the invention, a pulp is provided. The pulp has at least 7% hemicellulose and at least 2% Klason lignin. The pulp also has cellulose, wherein greater than 95% of the cellulose has an average D.P. of from greater than about 400 to about 1100.
In another embodiment of the invention, a meltblowing method for making a lyocell product is provided. The method includes extruding a cellulose solution of unbleached pulp through a plurality of apertures to produce continuous cellulose filaments. The method includes stretching the filaments with air.
According to the present invention, an unbleached pulp, having been modified to reduce its average degree of polymerization of cellulose, results in substantial increases in yield. The high amounts of hemicellulose and lignin contribute to this greater improvement in overall yield. This advantage, along with numerous related advantages, makes the lyocell product of this invention more desirable in comparison with previous technology.